18-Env-B9 Environmental Chemistry and Microbiology · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2016 — 04-Env-B9, Environmental Chemistry/Microbiology. 3 hours duration; closed-book exam (one 8.5×11" aid sheet, both sides, permitted; any non-communicating calculator permitted). The paper has two sections — Section 1: Chemistry (8 questions, 50 marks) and Section 2: Microbiology (12 questions, 50 marks) — twenty questions constitute the complete exam and all are answered below. Total examination mark 100.
Reference texts. Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) (water chemistry, disinfection, water/wastewater microbiology, indicator organisms); Metcalf & Eddy (Tchobanoglous, Stensel, Tsuchihashi & Burton), Wastewater Engineering: Treatment and Resource Recovery (5th ed.) (chemical unit processes, chemical phosphorus precipitation, biomass stoichiometry, activated-sludge microbiology, BOD/SRT/F–M); Guidelines for Canadian Drinking Water Quality (Health Canada); MWH's Water Treatment: Principles and Design (3rd ed.) (advanced treatment, UV disinfection, potable reuse).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Producing potable water from municipal wastewater (indirect/direct potable reuse) requires conventional secondary treatment to remove the bulk organic and nutrient load, followed by an advanced treatment train with multiple, independent barriers against pathogens and trace chemical contaminants before the water is fit to blend into a drinking-water supply.
Screening & grit removal. Coarse bar screens and a grit chamber protect downstream mechanical equipment and prevent abrasive/settleable inert solids from short-circuiting biological treatment.
Primary clarification. Quiescent settling removes ~50–60% of settleable/suspended solids and a portion of BOD by gravity before biological treatment, reducing the organic load the bioreactor must carry.
Nitrifying activated sludge. Aerobic biomass oxidizes dissolved/colloidal organics (BOD removal) and, at sufficient SRT, converts ammonia to nitrate (nitrification, see Question 15/16 below), which is essential because ammonia is toxic to aquatic life and a major chlorine/UV demand if carried into disinfection.
Secondary clarification. Separates the treated biomass (activated sludge) from the treated liquid stream; most solids are returned (RAS) to sustain the bioreactor, with the surplus wasted (WAS) to the solids-handling train.
Micro-/ultrafiltration. A physical membrane barrier removes residual suspended solids and essentially all bacteria and protozoan cysts, and pre-conditions the water (low turbidity, low fouling potential) for reverse osmosis.
Reverse osmosis. A semi-permeable membrane under pressure rejects dissolved salts, viruses, pharmaceuticals and other trace organics, providing the principal barrier against dissolved chemical contaminants and virtually all remaining pathogens.
UV / advanced oxidation (UV–H2O2). Provides an independent disinfection barrier (redundant with RO for pathogens) and destroys any trace organic compounds (e.g. NDMA, 1,4-dioxane) that can pass an RO membrane.
Stabilization & remineralization. RO permeate is corrosive (near-zero alkalinity/hardness); lime or CO2/limestone contactors restore alkalinity and pH to a stable, non-corrosive finished-water condition before blending into the potable supply.
